refactor: route _fs_namespace reads through getFsNamespace helper

[pablofullstory]

Summary

DLO had four hard-coded window._fs_namespace reads, which silently break for
sites whose FullStory snippet uses a custom namespace (set via the
data-fs-namespace script attribute) without also setting the _fs_namespace
global. This PR centralizes the lookup behind a single helper that mirrors the
resolution order used by @fullstory/browser-api.

• Add src/utils/fsNamespace.ts exporting getFsNamespace(win = window):
  data-fs-namespace on document.currentScript → _fs_namespace global → 'FS'.
  Self-contained (no @fullstory/browser-api dependency); defensive try/catch
  around document access; empty-string attribute falls through to the global.
• Replace the four direct reads with helper calls:
  • src/utils/logger.ts — FullStoryAppender.log
  • src/operators/fsApi/fsApi.ts — FSApiOperator.handleData
    (also tightens the throw message to Fullstory namespace is not a function)
  • src/target.ts — DataLayerTarget constructor's "is this the FS object?" guard
  • src/embed/init.impl.ts — attachDloFullStoryLifecycle

Behavior

• Existing call sites that only had the _fs_namespace global still work — the
  helper falls back to the global, which is the same floor the old code had.
• Sites that set data-fs-namespace="MyFS" on the FullStory snippet but do not
  set _fs_namespace now resolve correctly (this was the bug).
• document.currentScript is only meaningful while a script tag is being
  evaluated synchronously. DLO calls the helper from deferred callbacks
  (appender log, operator handleData, lifecycle hook), so the attribute path
  is opportunistic; the global remains the practical fallback. This matches how
  @fullstory/browser-api's own getFsNamespace is shaped.

Test plan

• npm test — 428 passing, including 8 new cases in test/fs-namespace.spec.ts
  covering: attribute wins over global, empty-string attribute is ignored,
  missing/null currentScript falls through, errors during document access
  are swallowed, default 'FS' floor, and undefined window.
• npm run lint — clean (only pre-existing no-console warnings remain).
• npm run build — clean.
• Existing specs that seed window._fs_namespace = 'FS' (logger.spec.ts,
  lifecycle-embed.spec.ts, operator-fsApi.spec.ts) still pass via the
  helper's global fallback.

---

Note

Low Risk
Localized integration refactor with preserved global/FS fallbacks and new unit tests; no auth or data-path changes.

Overview
Introduces getFsNamespace so every FullStory client lookup uses the same resolution order as @fullstory/browser-api: data-fs-namespace on document.currentScript (when available), then _fs_namespace, then 'FS'. Document access is wrapped in try/catch; an empty script attribute is ignored.

Replaces four direct window._fs_namespace reads in FullStoryAppender, FSApiOperator, DataLayerTarget, and attachDloFullStoryLifecycle with the helper. Sites that only set the global behave as before; sites that set a custom namespace on the snippet but omit _fs_namespace should now resolve the correct client function.

Adds test/fs-namespace.spec.ts for precedence, deferred/null currentScript, errors, and the default floor.

^{Reviewed by Cursor Bugbot for commit 3bb3717. Bugbot is set up for automated code reviews on this repo. Configure here.}

[ScottLNorvell]

let's say fs.js instead of @fullstory/browser-api since that's an internal package and this is theoretically an open source repo.

[ScottLNorvell]

LGTM! Just the one comment about removing the reference to @fullstory/browser-api. (which I'm sure that silly LLM put there hahhaha)

[ScottLNorvell]